The present invention relates to the production of reactive sintered nickel aluminide material, and more particularly to relatively low temperature and short duration reactive sintering under exothermic conditions of a shaped compact containing a powder mixture of elemental nickel and elemental aluminum in a corresponding stoichiometric atomic ratio to form directly the nickel aluminide compound Ni.sub.3 Al as a densified intermetallic compound material of relatively low porosity.
Intermetallic compounds are current candidates for use as high temperature, oxidation resistant materials finding application as turbine components, e.g. as turbine blades, etc., since advances in ceramics have failed to live up to expectations whereas those concerning superalloys have apparently been exhausted (see [1] C. C. Koch, C. T. Liu and N. S. Stoloff (eds.), High-Temperature Ordered Intermetallic Alloys, Materials Research Society Symposium Proceedings, vol. 39, Materials Research Society, Warrendale, PA, 1985; and [2] N. S. Stoloff, Inter. Metal Rev., 1984, vol. 29, pp. 123-135).
The intermetallic compounds based on aluminum have the attractive characteristics of low density, high strength, good corrosion and oxidation resistance, and relatively low cost. In some cases, the intermetallics exhibit the unique characteristic of improved strength with increasing temperature. Coupled with relatively high melting temperatures, these attributes make for ideal high temperature materials.
Powder metallurgy offers one approach for fabrication of complex shaped, high performance intermetallic compound alloys (see [3] W. M. Schulson, Inter. J. Powder Met., 1987, vol. 23, pp. 25-32; and [4] K. Vedula and J. R. Stephens, Powder Metallurgy 1986 State of the Art, W. J. Huppmann, W. A. Kaysser and G. Petzow (eds.), Verlag Schmid, Freiburg, West Germany, 1986, pp. 205-214). Powder metallurgy approaches include hot isostatic pressing (HIP), hot extrusion, injection molding and transient liquid phase sintering.
The pertinent prior art in this regard includes many U.S. Patents, as typified by the following.
U.S. Pat. No. 4,140,528 (Hebeisen et al) concerns hot workable nickel-base superalloy fully dense articles made for example by hot isostatic pressing (HIP) at 1900.degree.-2050.degree. F. (1038.degree.-1121.degree. C.) and 15,000 psi of -60 to -80 mesh prealloyed powder that had itself been produced by nitrogen gas atomizing of a molten metal mass of the desired superalloy composition including, besides a predominant content of Ni, small amounts of numerous elements such as Al and B.
U.S. Pat. No. 4,379,720 (Ray et al); U.S. Pat. No. 4,478,791 and U.S. Pat. No 4,606,888 (Huang et al); U.S. Pat. No. 4,609,528, U.S. Pat. No. 4,613,368 and U.S. Pat. No. 4,613,480 (Chang et al); and U.S. Pat. No. 4,612,165 (Liu et al); are directed to analogous additive element containing, especially boron doped, nickel aluminum alloys used as prealloys in powder metallurgy, plasma spraying, and the like.
U.S. Pat. No. 3,084,041 (Zegler et al) teaches the production of the extremely low temperature superconducting niobium tin compound Nb.sub.3 Sn of uniform stoichiometric composition, by melting a mixture of niobium powder and tin powder, the latter in excess of the stoichiometrical amount, at 900.degree. C. or higher for 7 hours or longer so as to form a prealloy, followed by solidification cooling, leaching of excess tin with concentrated hydrochloric acid for 12-24 hours and then sintering the Nb.sub.3 Sn in an inert atmosphere at 900.degree. C. or higher.
U.S. Pat. No. 3,260,595 (Maier et al) teaches the production of the extremely low temperature superconducting intermetallic compound vanadium-gallium V.sub.3 Ga, by precursor heating to about 700.degree. C. of a stoichiometrical mixture of vanadium powder and gallium powder, which results in an exothermic reaction causing the formation of needles of the precursor compound V.sub.2 Ga.sub.5, then grinding the needles to a powder and mixing such powder with additional vanadium powder in a specified stoichiometrical ratio, compressing the powder mixture, vacuum heating the compressed mixture at about 600.degree. C. for 30-60 minutes to remove adsorbed water and hydrogen, and finally sintering the so degassed mass under protective gas at about 1/2 atmosphere for about an hour at about 1300.degree. C. to produce a sintered body of V.sub.3 Ga. It is believed clear that one inherent problem with such a technique is gallium vaporization due to its high vapor pressure at temperatures above approximately 1100.degree. C.
U.S. Pat. No. 3,288,571 (Werner et al) teaches the production of pure form nuclear fuel uranium aluminides of the class UAl.sub.3 and UAl.sub.4.5, by heating a stoichiometrical mixture of aluminum and uranium (or uranium hydride) powders to a temperature as dictated by the U-Al system phase diagram to permit interdiffusion of the elements without melting the desired compound, using a hot pressing technique where UAl.sub.3 is to be formed.
U.S. Pat. No. 3,353,954 (Williams) concerns the formation of ceramic articles such as nuclear fuel elements, containing in situ intermetallic compounds such as borides, aluminides including NiAl, silicides, etc., as a ceramic matrix for other compounds as diluents such as alumina, etc., by heating under compacting pressure a particular mixture of the ingredients for in situ reaction and interbonding thereof.
U.S. Pat. No. 2,877,113 (Fitzer) concerns the reaction of nickel and aluminum powders in liquid mercury at 370.degree.-750.degree. C. to form an alloyed nickel-aluminum compound containing 17-35% Al such as NiAl.sub.3 (as distinguished from Ni.sub.3 Al) which upon being freed of adhering mercury can be used as a prealloy for sintering to form shaped bodies.
U.S. Pat. No. 3,653,976 (Miller et al) concerns a classic brute force approach for the production of the intermetallic compound nickel aluminide NiAl as a prealloy, by adding aluminum to melted nickel in stoichiometric quantity in an argon atmosphere of 5 psig, which results in an exothermic reaction that increases the furnace temperature from 2800.degree. F. (1538.degree. C.) to about 3100.degree. F. (1704.degree. C.), followed by solidification cooling, powdering and compression sintering of the prealloy in a vacuum to form a shaped body such as a turbine rotor blade.
U.S. Pat. No. 2,755,184 (Turner Jr., et al) concerns compacting and then sintering a powder mixture of the precursor compound NiAl and sufficient metallic nickel to yield the desired compound Ni.sub.3 Al at a temperature not substantially in excess of the solidus temperature (2525.degree. F.; 1385.degree. C.) of the compound Ni.sub.3 Al, i.e. first above the melting point of the nickel such as at 2600.degree.-2650.degree. F. (1427.degree.-1454.degree. C.) for 5-10 minutes and then at 2300.degree.-2550.degree. F. (1260.degree.-1399.degree. C.) for 1-25 hours in a non-oxidizing atmosphere, to permit solid state diffusion, thereby producing Ni.sub.3 Al to the exclusion of NiAl. It is stated that mere heating of a mixture of metallic aluminum and metallic nickel in proper atomic proportions does not produce the desired intermetallic compound due to the formation of an oxide scum on the aluminum which prevents reaction thereof with the nickel, and that formation of the compound Ni.sub.3 Al requires special technique because of the very restricted area of the nickel-aluminum phase diagram in which the compound exists as a stable phase. It is stated that the produced Ni.sub.3 Al sintered compact can be machined, has a high hot strength, is tough and relatively ductile, and can withstand oxidizing temperatures of 1600.degree.-2000.degree. F. (871.degree.-1093.degree. C.) without significant loss due to oxidation.
It is clear from the foregoing that the concept of reactive sintering and similar processes have been applied to the intermetallic formation of several compounds in the past (e.g., see U.S. Pat. No. 2,755,184; U.S. Pat. No. 2,877,113; U.S. Pat. No. 3,084,041; U.S. Pat. No. 3,260,595; U.S. Pat. No. 3,288,571; U.S. Pat. No. 3,353,954; and U.S. Pat. No. 4,613,368, supra). Indeed, the process of combustion synthesis is similar, but involves a greater heat of formation for the compounds (see [5] O. Yamada, Y. Miyamoto and M. Koizumi, Bull. Amer. Ceramic Soc., 1985, vol. 64, pp. 319-321).
However, success in the pertinent formation of Ni.sub.3 Al in particular has apparently only been achieved per U.S. Pat. No. 2,755,184 to Turner Jr. et al and only by way of a high temperature treatment of mixed powders of the prealloy NiAl and elemental nickel, with the temperature being in the order of 1300.degree. C., such that the process involved is essentially one of solid state homogenization and not reactive sintering. On the other hand, the production of NiAl per U.S. Pat. No. 2,877,113 to Fitzer involves the reaction of Ni and Al in a mercury amalgam at temperatures as high as e.g. 700.degree. C., which leads to the formation of NiAl powder that is subsequently compacted and sintered at temperatures above 1350.degree. C. Neither of these processes is concerned with the direct production of the Ni.sub.3 Al intermetallic compound from mixed elemental powders, and both require substantially high final sintering temperatures.
U.S. Pat. No. 3,084,041 to Zegler et al, U.S. Pat. No. 3,260,595 to Maier, U.S. Pat. No. 3,288,571 to Werner et al, and U.S. Pat. No. 3,353,954 to Williams, are believed to be less pertinent in covering other intermetallic systems such as Nb.sub.3 Sn, V.sub.3 Ga, UAl.sub.3 and MoSi.sub.2, as the case may be. The formation of these other compounds is achieved by processing mixed powders, involving steps of reacting, pulverization or grinding, compaction and sintering, and variations including hot pressing and pressure assisted sintering. In each case, stoichiometry is important and is often achieved using an excess of the more volatile ingredient or intermediate chemical leaching to remove unreacted constituents. Again, these known procedures concerning reactive sintering of intermetallic compounds rely on the reaction to form a compound powder, but use subsequent separate steps to densify the compound.
In recent research on Ni.sub.3 Al, these various approaches have apparently been abandoned in favor of gas atomization and hot isostatic compaction (see [1] C. C. Koch, C. T. Liu and N. S. Stoloff (eds.), High-Temperature Ordered Intermetallic Alloys, supra; [2] N. S. Stoloff, Inter. Metal Rev., supra; [3] W. M. Schulson, Inter. J. Powder Met., supra; and [4] K. Vedula and J. R. Stephens, Powder Metallurgy 1986 State of the Art, supra). The success of this last noted approach is clearly established, yet there are the considerable drawbacks thereto of long process cycles, high process temperatures and significant attendant expense.
There is a clear need for an approach such as one involving reactive sintering, that circumvents the various explicit and implicit problems associated with the above discussed techniques, and permits use conveniently of commercially available elemental powders, comparatively low processing temperatures and short process cycles, and classic press and sinter technology.